In inline vacuum coating installations the substrates to be coated are transported to several coating sources and are coated by these sources. For example, glass plates are conveyed in their horizontal position on transport belts or transport rollers. Since the substrates are only resting in contact on individual rollers, very thin glass panes can sag in the sections between the rollers. The reliable transport of the panes on such a system can consequently not be ensured. The substrates can also be transported in the vertical or nearly vertical position. In order to be able to hold them reliably in this position, they must be guided at their upper and lower edges. But on the guide rails of the transport system particles can be generated, which, depending on the quality requirements made of the coating, can lead to rejected items if such arrive on a substrate. This hazard develops especially through the upper guide rails of the transport system, since the particles with high probability fall from them onto the substrate.
It is known to secure or clamp work pieces under atmospheric pressure by disposing them on a porous sintered metal plate, on whose backside a vacuum is generated. Through the porous vacuum plates the work pieces become attached by suction and secured (Catalog “Vakuum Spanntechnik”/Horst Witte Gerätebau). In a known transport system flat substrates are placed onto porous sintered metal plates of this type and through the porous plate compressed air is blown from below, which forces the air to flow through the fine pores of the sintered metal plate. This makes the substrate hover on an air cushion (device by MANZ Automation).
Such transport systems cannot be utilized in vacuum. The enormous surface resulting from the porosity of the vacuum plate or the sintered metal plate adsorbs gas or water from the air if the vacuum installation is flooded. The adsorbed material is desorbed again in the subsequently established vacuum, whereby an additional and not controllable quantity of gas is introduced. This quantity of gas alone would be capable in the vacuum to allow a glass pane to be lifted. In addition, such a gas load also impairs reactive sputter processes, since the additional gas quantity flowing in under lack of control changes the ratio of reactive gas to inert gas and nonreactive coatings are contaminated by foreign atoms.
A transport system is furthermore known in which the substrates, for example CDs, hover on a gas cushion (CD production installation of Leybold AG, around 1990). In this system, which is applied under atmospheric pressure, the gas cushion is generated thereby that through bores in an approximately 120 mm wide and approximately 1 m long flat metal plate compressed air is blown from below against the CDs. These bores are spaced apart from one another at a distance of approximately 5 cm and are disposed in rows at a distance of approximately 5 cm. Some of the bores are not perpendicular, but rather are set in at an angle to the metal surface, which lends the blown-out air a component in the direction of transport and the CDs are moved forward. The bores have a diameter of somewhat less than 1 mm. In order to reduce the consumption of purified compressed air, individual sections of the transport path are continuously connected with a compressed air supply. Behind the CD the compressed air is subsequently switched off again in order to ensure that, on the one hand, the CD is moved without contacting the metal surface, but, on the other hand, nonrequired sections of the transport path do not unnecessarily waste compressed air.
This system can also not be utilized in vacuum, since the number and the cross section of the bores would cause too great a gas throughput. It has been found that, in contrast to transport systems operated under atmospheric pressure, the gas flow in a transport system operated in vacuo must be selected with much deliberation depending on the chamber pressure, since otherwise the glass pane can be excited into oscillation.
A work piece carrier for a pane-form work piece is already known, which is subjected to surface treatment, especially in a vacuum installation (DE 39 43 482). This work piece carrier comprises a work piece bearing surface with a multiplicity of exit openings communicating with a distribution volume. Connected to this distribution volume is a gas supply, which supplies a gas to form a heat-transferring gas cushion between bearing surface and work piece. The distribution volume is formed by a groove, which is worked into a plate and covered by a cover plate. In addition to the exit openings in the bearing surface, a multiplicity of aspiration openings are provided, each of which extends next to the groove through the plate and cover plate and communicates with a gas aspiration volume to which a gas draw-off line is connected. The gas impinging onto the work piece in this case does not serve for the purpose of keeping the work piece away from a contact.
The invention is based on the task of providing a transport system for substrates to be coated, which can also be applied in a vacuum and in which no abrasion occurs on the substrates.
This task is solved according to the arrangement according to the present invention.